Wind turbines for generating electricity are known and are being deployed in increasing numbers to provide a sustainable electricity supply. The maximum power which a wind turbine can generate is proportional to the swept area of its blades, which swept area is approximately proportional to the square of the lengths of the blades. There is a desire to increase the size of wind turbines so as to increase the maximum power which they are capable of providing and so as to reduce the cost of the supplied energy. However, linear scaling of output power cannot be achieved simply by linearly scaling the wind turbine components.
For example, if the length of each blade is doubled, the swept area is increased four-fold. However, the weight of each blade is generally proportional to the cube of the length so that the weight is increased eight-fold.
Wind turbines are known in which blades are fixed to a hub which drives a generator via a shaft connected to that hub. Examples of this arrangement are disclosed in WO 03/098034A1, DE 10311025A1, US 2006/0251516A1, EP 0058791A1 and U.S. Pat. No. 6,951,443B1.
Generators driven by axles have typically required the use of a gearbox to increase the rotary speed so as to provide more efficient generation of electricity. However, gearboxes are inconvenient in that they add to the cost, weight and servicing requirements of wind turbines. Direct drive generators have been used and examples of such arrangements are disclosed in DE 10255745A1 and U.S. Pat. No. 6,285,090B1.
WO 02/099950A1 discloses an electrical generator of this type. The wind turbine comprises a rotor whose rim is connected to a hub by tension members so as to resemble a spoked bicycle wheel. The hub is driven by an axially spaced wind turbine blade arrangement.
CA26265452A1 discloses a wind turbine with a direct drive generator. The blades of the turbine rotor are fixed to an annulus, which is connected to a hub by rods extending radially from the hub and disposed in a common plane perpendicular to the axis of rotation. The blade forces are resolved at the annulus, to which the inner ends of the blades are connected. The annulus must therefore be fabricated with sufficient strength to withstand these forces. For an annulus having a radius about the axis of rotation of the order of 12 meters and a cross-sectional diameter of the order of 3.5 meters, manufacture is relatively expensive and the annulus is of considerable weight. Transportation and assembly of the wind turbine therefore tend to be expensive.
U.S. Pat. No. 7,042,109B2 discloses various arrangements of wind turbines in which a turbine rotor drives an ironless ring-type generator directly or through a gearbox. In the arrangement shown in FIG. 3 of this document, the blades extend within a rotor hub to the hub bearings and a generator rotor is fixed to the hub. Components parallel to the axis of rotation of forces acting on the blades are communicated to the bearings and also to the generator rotor. Torque produced by the blades is transmitted to the generator rotor.
Wind turbines with the blades fixed to a hub and braced by a rope or cable arrangement are also known. In such turbines, the hub again drives a generator via an axle and typically via a gearbox. Wind turbines of this type are not suitable for relatively high power generation as the swept area cannot be increased sufficiently while maintaining the integrity of structure.
Other examples of wind turbines are disclosed in GB 1511948A1, WO 97/13979A1, U.S. Pat. No. 4,330,714A, U.S. Pat. No. 4,350,895A, WO 86/02701A1, WO 2007/135391A2, US 2005/200134A1, U.S. Pat. No. 5,118,255A, U.S. Pat. No. 2,050,129A, DE 19606359A1 and US 2008/124216A1.